Inline fmt lints for `redirects/`

I couldn't find any documentation of how `redirects/` dir is being used,
but fixed inlined format args for consistency

redirects/choosing-your-guarantees.md

@@ -7,7 +7,7 @@
 
 ```rust
 let b = Box::new(5);
-println!("b = {}", b);
+println!("b = {b}");
 ```
 
 ---

redirects/iterators.md

@@ -11,7 +11,7 @@ let v1 = vec![1, 2, 3];
 let v1_iter = v1.iter();
 
 for val in v1_iter {
-    println!("Got: {}", val);
+    println!("Got: {val}");
 }
 ```
 

redirects/lifetimes.md

@@ -11,7 +11,7 @@
                           //      |
     let r = &x;           // --+--+-- 'a
                           //   |  |
-    println!("r: {}", r); //   |  |
+    println!("r: {r}");   //   |  |
                           // --+  |
 }                         // -----+
 ```

redirects/mutability.md

@@ -6,9 +6,9 @@
 
 ```rust
 let mut x = 5;
-println!("The value of x is: {}", x);
+println!("The value of x is: {x}");
 x = 6;
-println!("The value of x is: {}", x);
+println!("The value of x is: {x}");
 ```
 
 ---